Electrolyte membranes are known in the art, such as, for example, for use in battery structures and fuel cells. For example, U.S. Patent Application Publication No. 2009/0081512 describes zirconia-based ceramic electrolyte membranes and related methods of making by laser micromachining techniques. Other electrolyte membrane materials are based on lithium metal phosphates (“LMP”) such as Lithium Titanium Phosphate (“LTP”), and may comprise LTP wherein a fraction of the titanium is replaced by Aluminum (“LATP”). It is well known that LMP electrolyte membranes are distinct from, and have different properties than, zirconia-based electrolyte membranes.
Thin LMP membranes may be desired in certain applications, for example to improve the membrane conductance, which is directly proportional to the membrane thickness. Conventional LMP-based membranes typically have conductivity of about 5×10−4 S/cm, and a conductance of approximately 0.02 S. Membranes with conductance of about 0.05 S, or more preferably about 0.1 S or higher, are desirable in certain applications. Furthermore, it is known that these targets can be achieved by reducing the membrane thickness, for example to about 100 μm (0.05 S) or about 50 μm (0.1 S).
However, although thinner LMP membranes may be more desirable, for example in battery structure applications where a thinner membrane provides lower impedance and thus, lower internal resistance and higher power capability, it is generally difficult to make thin membranes having sufficiently precise dimensions with known methods, for example due to limits of mechanical cutting technology. Additionally, thin LMP membranes are typically fragile, and therefore difficult to fabricate and handle. For example, conventional mechanical cutting processes limit LMP membrane thickness to about 200 μm. This is particularly true when the membrane is unsupported, i.e., where the membrane is not integrated into a multi-layer structure wherein some other layer provides mechanical support.
In addition to the foregoing, polycrystalline electrolyte membranes typically have large grain boundary resistance compared to intra-grain resistance. Therefore, typical ceramic electrolyte membranes are often large-grained in order to minimize grain boundary effects. However, thin electrolyte membranes with large grains are typically weak and fine-grained ceramic membranes are mechanically superior to conventional ceramic membranes. Also, the mechanical properties of thin membranes can be severely degraded by edge defects and wrinkling that occurs during processing. For example, edge defects may be produced where the cutting process introduces microstructural features that may become points of stress concentration, thus reducing strength. Defects such as gross accumulation of melted material or voids resulting from substantial movement of melted material are undesirable in at least certain embodiments, as they may degrade mechanical properties if they are significant enough to influence stress distribution within the membrane. Further, it can be difficult, using conventional mechanical methods, to prepare an LMP electrolyte membrane having precise dimensions, due to shrinking during firing.
There is, therefore, a need to provide mechanically strong, thin, fine-grained ceramic lithium ion electrolyte membranes with high quality, defect-free edges.